Re: [PATCH v2] drm/dp/mst: fix OOB reads in remote DPCD/I2C sideband reply parsers

[lyude@redhat.com]

Reviewed-by: Lyude Paul <lyude@redhat.com>

Will push to drm-misc in just a moment

On Sun, 2026-05-10 at 20:17 +0000, Ashutosh Desai wrote:
> drm_dp_sideband_parse_remote_dpcd_read() reads num_bytes from the raw
> message and then unconditionally does:
> 
>   memcpy(bytes, &raw->msg[idx], num_bytes);
> 
> without checking that idx + num_bytes <= raw->curlen. raw->msg[] is
> 256 bytes; if a malicious or misbehaving MST hub sets num_bytes
> larger
> than the remaining payload, the memcpy reads past the received data
> into whatever follows in raw->msg[].
> 
> drm_dp_sideband_parse_remote_i2c_read_ack() has the same flaw (noted
> with a /* TODO check */ comment since the code was introduced).
> 
> Fix both functions by using a single combined check
> (idx + num_bytes > curlen) before each memcpy. Since num_bytes is u8,
> it is always >= 0, so this strictly subsumes the simpler idx > curlen
> form and no separate step is needed.
> 
> Cc: stable@vger.kernel.org
> Signed-off-by: Ashutosh Desai <ashutoshdesai993@gmail.com>
> ---
> Changes in v2:
> - Drop separate idx > curlen check; idx + num_bytes > curlen with u8
>   num_bytes (always >= 0) strictly subsumes it (Lyude Paul)
> 
>  drivers/gpu/drm/display/drm_dp_mst_topology.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c
> b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> index 170113520a43..9416a48804c8 100644
> --- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> @@ -871,7 +871,7 @@ static bool
> drm_dp_sideband_parse_remote_dpcd_read(struct drm_dp_sideband_msg_rx
>  		goto fail_len;
>  	repmsg->u.remote_dpcd_read_ack.num_bytes = raw->msg[idx];
>  	idx++;
> -	if (idx > raw->curlen)
> +	if (idx + repmsg->u.remote_dpcd_read_ack.num_bytes > raw-
> >curlen)
>  		goto fail_len;
>  
>  	memcpy(repmsg->u.remote_dpcd_read_ack.bytes, &raw->msg[idx],
> repmsg->u.remote_dpcd_read_ack.num_bytes);
> @@ -907,7 +907,9 @@ static bool
> drm_dp_sideband_parse_remote_i2c_read_ack(struct drm_dp_sideband_msg
>  		goto fail_len;
>  	repmsg->u.remote_i2c_read_ack.num_bytes = raw->msg[idx];
>  	idx++;
> -	/* TODO check */
> +	if (idx + repmsg->u.remote_i2c_read_ack.num_bytes > raw-
> >curlen)
> +		goto fail_len;
> +
>  	memcpy(repmsg->u.remote_i2c_read_ack.bytes, &raw->msg[idx],
> repmsg->u.remote_i2c_read_ack.num_bytes);
>  	return true;
>  fail_len: